2 periods in a month?!

In March I had a period 1st-6th and then 24th-28th. I phoned doctor yesterday and he said he's expecting me to get a period at the end of April but that can't be right because my period calculator says it should start on 16th. I took a pregnancy test today which was negative. So I still don't know why I had 2 periods in March when I usually have a 28 day cycle?? I'm confused x